There is a type of a motor drive apparatus that converts, by an inverter, direct current (DC) electric power supplied from a DC power supply such as a battery and uses the converted electric power to drive and control a motor. Such a motor drive apparatus generally includes a smoothing capacitor that is connected between a power supply line and an earth line for stabilizing a DC voltage input to the inverter.
In the above motor drive apparatus, for safety reasons, it is required to surely discharge the electric charge stored in the smoothing capacitor when electric power feeding to a load stops. Therefore, in many cases, a discharge resistance for discharging the smoothing capacitor is connected between the power supply line and the earth line in parallel with the smoothing capacitor.
In order to discharge the smoothing capacitor as soon as possible, it is required to minimize a resistance value of the discharge resistance. On the other hand, if the resistance value of the discharge resistance is small, a current flowing through the discharge resistance is increased when a power supply device supplies electric power to the load. Therefore, power consumption of the discharge resistance is increased.
Japanese Patent Laying-Open No. 7-170776 discloses a method for discharging electric charge in a main circuit of an inverter, which is designed for solving such contradictory problems. The main circuit of the inverter disclosed in this document includes: a rectifier connected to a motor and rectifying an output of an alternating current (AC) power supply; a contactor for turning on/off a DC voltage output of the rectifier; a capacitor for smoothing the DC voltage output of the rectifier; a switching element for converting the DC voltage to a three-phase pulse voltage obtained by nearly sinusoidal PWM (Pulse Width Modulation) and supplying the pulse voltage to the motor; and a discharge resistance connected in parallel with the capacitor. At the time of discharge of the capacitor, the contactor is first turned off, and then, the switching element is turned on. As a result, a current in the main circuit flows through an exciting winding of the motor, and therefore, the capacitor can be discharged rapidly even if a resistance value of the discharge resistance remains high.
Since the discharge resistance is provided to the inverter, problems such as an increase in the number of components required for the motor drive apparatus and the necessity for an installation step arise. These problems become an impediment to cost reduction of the motor drive apparatus.